This invention relates generally to optical signal processing systems and more particularly to transversal filters for use in processing wideband signals.
Communications systems and radar systems commonly use filters in the processing of electromagnetic signals detected by system receivers. For example, in a radar system the detected return signals usually must be filtered in order to extract a reliable signal to determine the position of the target. Nonrecursive finite impulse response filters are typically used for this purpose, such as a transversal filter, also known as a tapped delay-line filter.
A transversal filter includes multiple stages of time delays through which the signal to be filtered passes, with the output of each stage tapped to provide a respective signal to a trimmer amplifier that selectively adjusts the amplitude of that tapped signal according to an algorithm that produces a desired performance or signal output. Conventional transversal filters typically have electrical components to process the filter input signal, and a major disadvantage is the large number of electrical delay elements and amplifiers that are required. Such electrical filters are complex to assemble and maintain, and the number and nature of components in the trimmer circuit present numerous potential failure modes. Further, signal losses in the delay components coupled in sequential stages may reduce the filter's operational sensitivity.
Opto-electronic signal processing has been applied in a variety of communications and radar systems. For example, for phased array radar systems, opto-electronic processing can be used to generate either time delayed or phase shifted optical signals. Use of some optical architectures, such as those having deformable mirror devices (DMDs) to direct light beams along desired paths for processing, have also been applied to a transversal filter. See E. Toughlian, H. Zmuda, "Variable Time Delay for RF/Microwave Signal Processing", SPIE Optical Technology for Microwave Applications V (1991), vol. 1476, pp 116-19. DMDs, however, necessarily involve control of displacement of a mirror to achieve the desired processing of the optical signal, and this type of system is less rugged and potentially prone to calibration errors given the large number of mirrors with respective small displacements required in such systems.
Optically processing the differentially time delayed signals in an optical transversal filter provides superior performance over conventional electronic-only processing of signals by reason of, for example, low loss wide bandwidth signal processing. Ideally, a transversal filter is able to effectively process a wide bandwidth input signal, is compact, relatively immune to undesirable electromagnetic radiation, and straightforward to fabricate, operate, and maintain. Such a filter also desirably has inertialess, motion-free operation that is readily adapted for reliable operation in a number of environments, such as aircraft or ships.
It is accordingly an object of this invention to provide a high performance optical transversal filter for filtering wide bandwidth signals.
It is a further object of this invention to provide an optical based amplitude control apparatus that is relatively compact, lightweight, and inertialess.